Macrocyclic metal chelates using 1,4,7,10-tetraazacyclododecane-N,N′,N″,N′″-tetraacetic acid (DOTA) may be administered to patients and animals to create or enhance contrast in biomedical imaging studies. Examples include paramagnetic Gd(III)-DOTA for magnetic resonance imaging (MRI), radioactive Tc-DOTA for single photon emission computed tomography (SPECT), and fluorescent Eu(III)-DOTA for fluorescence imaging. DOTA provides flexibility for synthetic derivatization that can change the pharmacokinetics of a chelate in vivo. This facilitates providing additional imaging information about biological processes.
Metal-DOTA chelates have been conjugated to peptides to affect the pharmacokinetics of a metal-DOTA imaging agent in vivo. This facilitates acquiring additional information about biological processes at the molecular level. A variety of peptidyl ligands have been employed for these molecular imaging studies, including peptides that bind to specific cell surface receptors, peptides that penetrate cell membranes, peptides that nonspecifically interact with the extracellular matrix, and peptide homopolymers that alter renal clearance rates.
The carboxylates of DOTA have conventionally been conjugated to the tertiary amines of peptides, including the N-terminal amine, the side chain amine of lysine, and unnatural amine-derivatized amino acid residues (e.g., p-NH2-phenylalanine). Other DOTA derivatives have been devised for conjugation to peptide amino groups, (e.g., succinimide DOTA derivatives) and isothiocyanato DOTA derivatives. However, coupling DOTA only to the N-terminus of a peptide amine can limit synthesis methodologies and may compromise the utility of the peptidly contrast agent for molecular imaging.